A process for producing low-biuret urea

ABSTRACT

A process for purifying a urea-containing aqueous stream, such as the aqueous stream from the recovery section of a urea plant, comprising a step of removing biuret from the urea-containing stream by reverse osmosis in one or more reverse osmosis stages.

DESCRIPTION Field of the Invention

The invention relates to the field of production of urea or urea-basedproducts. The invention relates in particular to the removal of biuretfrom aqueous solutions of urea.

Prior Art

Urea is synthesized industrially by reacting ammonia and carbon dioxide.An overview of the related processes can be found in the Ullmann'sEncyclopaedia of Industrial Chemistry, Wiley-VCH Verlag.

Urea is typically produced by reacting ammonia and carbon dioxide in aurea synthesis section at a suitable urea synthesis pressure, to form aurea-containing reaction effluent. This effluent is essentially anaqueous solution of urea containing unreacted ammonia and carbondioxide, mostly in the form of ammonium carbamate. In a well knownembodiment the synthesis section includes a reactor, a stripper and acondenser forming a high-pressure loop. The reactor effluent is heatedin the stripper, possibly with the help of a gaseous stripping agent, toremove a gaseous stream containing ammonia and carbon dioxide. Thisgaseous stream emerging from the stripper is condensed in the condenser,possibly with the help of a solution recycled from the recovery section.The so obtained condensate is recycled to the reactor. The famousStamicarbon CO₂-stripping process uses gaseous CO₂ as a stripping agent.Another stripping process uses gaseous ammonia as a stripping agent.

The reaction effluent is typically processed in a recovery section,including one or more recovery stages at a recovery pressure lower thansaid synthesis pressure, to remove the unreacted ammonia and carbondioxide from the reaction effluent and to obtain a urea aqueous solutionconsisting essentially of urea and water. A recovery stage for exampleincludes heating the solution to obtain dissociation of carbamate andcondensing the so obtained vapours into a carbamate-containing recyclesolution. This solution may be recycled to the synthesis section, e.g.to the condenser of the synthesis loop.

The aqueous solution withdrawn from the recovery section typicallycontains 60% to 90% urea by weight. This solution may be processed toremove water and obtain a highly concentrated solution or a urea melt tofeed a granulation section or prilling section where solid urea isproduced. It is known that a granulation section requires an input ureamelt containing at least 96% of urea by weight; a prilling sectionrequires a urea melt of at least 99.7% of concentration.

Another use of urea of economic interest is the production of an aqueoussolution of urea for use in the selective catalytic reduction of NOxfrom exhaust gas (SCR solution). The content of urea in a SCR solutionmay vary; a solution for use in the automotive field, so called dieselexhaust fluid (DEF), typically contains 30 to 35% by weight, preferably31.8% to 33.2% and most preferably 32.5% of urea. To this purpose, theurea solution from the recovery section may be diluted with water untilthe target concentration of urea is reached as disclosed for example inEP 1 856 038.

The solution from the recovery section consists mostly of urea and waterbut also contains some impurities. One of the most problematic impurityis biuret. The formation of biuret occurs practically in every stage ofurea production and is promoted by residence time at high temperature.

Biuret has the formula H₂N—CO—NH—CO—NH₂ and forms when urea is heatedabove its melting point according to the reaction: 2 urea→biuret+NH₃.

The quality requirements of the final product in terms of maximumacceptable content of biuret are stringent and difficult to achieve. Atypical target for solid urea is 0.9% wt. or less, calculated as kg ofbiuret per kg of solid product. This target is typically required foruse of the solid urea as soil fertilizer; a foliar-grade fertilizer mayhave a significantly lower limit of acceptable biuret. As explainedabove, the starting material for production of solid urea is an aqueoussolution containing 60 to 80% wt. urea which is treated to remove waterand the so obtained highly concentrated melt is granulated or prilled.

It is difficult to maintain such a low biuret content in the finalproduct. As the content of biuret obviously increases when water isremoved, a producer of urea may be forced to lower the concentration ofthe urea melt sent to the granulation or prilling section in order tomeet the maximum biuret in the solid urea. However the granulationprocess or prilling process are strongly affected by any additionalcontent of water in the urea melt feed.

Similar requirements in terms of maximum biuret are encountered in theproduction of SCR solutions. For example the maximum acceptable biuretin the DEF is typically 0.3% wt, as prescribed e.g. by the DIN V70070Norm. Taking into account the 30 to 35% concentration of urea in theDEF, this means that solid urea dissolved to produce the DEF shall notexceed 0.9% of biuret. If the DEF is produced directly by diluting a 70%solution, the starting solution must not exceed 0.6% of biuret (allpercentages by weight).

The control of biuret is also complicated by the fluctuations of theproduction. For example when a urea plant runs at a partial load theresidence time of urea melt at high temperature may be longer and,consequently, more biuret is formed.

A known process to obtain a low-biuret solid urea from the aqueoussolution withdrawn from the recovery section is concentration bycrystallization. In this process, crystals of highly pure urea areobtained and subsequently melted to produce a urea melt. Howevercrystallization is expensive. It requires centrifugation to separate thecrystals from the solution; careful handling of crystals e.g. bypneumatic means; a melter to melt the crystals. All the above requiresitems which are expensive and difficult to operate.

There is the need to provide a process for obtaining low-biuret ureasolution which is cost-effective, easy to implement and manage,efficient also at partial loads, and compatible with a concentrationsection based on evaporation.

SUMMARY OF THE INVENTION

The purpose of this invention is to overcome the above describeddrawbacks of the prior art. A goal of the invention is to provide acost-effective and practical process for removing biuret from an aqueoussolution of urea. Particularly, a goal of the invention is to provide aprocess for removal of biuret which is applicable to a process forproducing urea including concentration by evaporation. Still anothergoal is to provide a process for making urea with a low content ofbiuret to meet the nowadays stringent quality requirement. Another goalis to provide a process for removing biuret which is effective also atpartial loads of a urea plant.

In one application, the invention aims to produce solid urea with nomore than 0.9% by weight of biuret, preferably no more than 0.7% wt,with a process including concentration by evaporation and subsequentgranulation or prilling. With reference to another preferredapplication, one aim of the invention is a process for producing a SCRsolution with a low content of biuret although the initial solid productto be dissolved or the urea solution to be diluted contains high levelbiuret. Particularly, one aim is to produce a SCR solution in accordancewith the quality requirements of the DIN 70070 Norm, including no morethan 0.3% biuret by weight.

The above aims are reached with a process according to the claims. Thedependent claims disclose preferred embodiments.

The invention is based on the innovative idea to remove biuret from aurea-containing aqueous solution by means of reverse osmosis.

The reverse osmosis (RO) is a process known in itself which involves thepassage of an aqueous stream through a semi-permeable membrane andseparation of a permeate from a retentate. In the present invention, areverse osmosis process through a semi-permeable membrane separatesbiuret from the aqueous solution of water and urea.

The applicant has experimentally tested that the molecule of biuret canbe separated efficiently from a urea solution in a membrane based ROprocess. A preferred membrane for carrying out the invention is athin-film composite (TFC) membrane. Preferably the process of theinvention is carried out with a membrane having a nominal retentioncoefficient equal to or greater than 99.0% on NaCI.

The invention is preferably applied to the aqueous urea solutionwithdrawn from the recovery section of a urea plant, which consistsessentially of urea and water.

An aspect of the invention is a process comprising:

-   -   reacting ammonia and carbon dioxide under urea-forming        conditions and urea synthesis pressure in a urea synthesis        section to form a urea-containing reaction effluent;    -   processing said urea-containing reaction effluent in a recovery        section, including one or more recovery stages at a recovery        pressure lower than said urea synthesis pressure, to remove        unreacted ammonia and carbon dioxide from the reaction effluent        and obtain a urea aqueous solution;    -   purifying said urea aqueous solution to remove biuret with a        process of reverse osmosis.

One of the aspects of the invention is the hindsight that biuret can beremoved from the aqueous urea solution, with a reverse osmosis process,before the urea solution is sent to a concentration section for theproduction of solid urea. Accordingly an aspect of the invention is alsoa process for producing solid urea which includes the steps of taking anaqueous solution of urea from the recovery section of a urea synthesisplant, optionally after storage of the solution in a tank, removal ofbiuret with a reverse osmosis process; subsequent concentration of theso obtained low-biuret solution to remove water; processing of the soobtained concentrated solution or melt to obtain a solid urea product,e.g. by granulation or prilling.

In another interesting application, an aqueous solution of urea for SCR(SCR solution), preferably containing 30 to 35% urea by weight, ispurified from biuret with a reverse osmosis process. Said aqueoussolution may be obtained by dissolving solid urea in water or simply bydiluting a more concentrated solution (e.g. the solution from therecovery section) with water.

Another aspect of the invention is a plant for the production of ureaaccording to the claims.

The process of the invention does not significantly separate urea fromwater. Accordingly a solution with a reduced content of biuret obtainedwith the process of the invention may have the same or substantially thesame water to urea ratio (kg/kg) as the input solution. The removal ofbiuret alone without affecting the water to urea ratio can be achievedwith an appropriate difference of pressure across the membrane. Saiddifference of pressure denotes the difference of pressure between thepermeate side and retentate side of the membrane and is commonly termeddelta-pressure.

The delta-pressure across the membrane is greater than a first osmoticpressure Π₁ and lower than a second osmotic pressure Π₂ wherein: thefirst osmotic pressure Π₁ is the osmotic pressure that can be calculatedfor the aqueous urea solution assuming biuret is the solute and theurea/water mixture is the solvent; the second osmotic pressure Π₂ is theosmotic pressure that can be calculated for the aqueous urea solutionassuming urea is the solute and water is the solvent. By selecting adelta pressure in this range, a significant amount of biuret can beremoved obtaining a permeate with substantially the same water to urearatio as the input solution.

Preferred Embodiments

In this description and in the claims, all percentages are given inweight unless otherwise specified.

The reverse osmosis process of the present invention is preferablyperformed with the input urea-containing stream having a temperature of60° C. to 90° C., preferably 70° C. to 80. Particularly preferably, thetemperature of the input stream is 70° C. to 75° C.

The RO process may be performed in a single RO stage or, morepreferably, in a plurality of RO stages in a cascade. Each stagepreferably operates within the above mentioned temperature ranges. Theterm cascade denotes that at least one of the permeate, the retentate orboth of them of at least one stage is/are further processed in one ormore subsequent stages.

In multiple-stage embodiments the various preferred embodiments of theprocess, which are disclosed in this description, may be applied to atleast one stage or, preferably, to all stages.

The difference of pressure across the RO stage, or each RO stage in caseof multiple stages, is preferably 30 bar to 70 bar, more preferably 35bar to 50 bar and more preferably 40 bar or around 40 bar. Thepermeability of one stage may be, for example, around 10 litres per hourand per m².

A reverse osmosis stage produces a permeate and a retentate. Thepermeate is the purified solution containing less biuret than the inputsolution; the retentate contains the biuret removed from the inputsolution and therefore has a relatively high content of biuret,typically more than 1% by weight.

In a multiple-stage embodiment, the input solution may be processed in afirst RO stage obtaining a first permeate and a first retentate. Thefirst permeate may be processed through a first set of one or moresubsequent RO stages wherein the permeate of the n-th stage is sent tothe (n+1)-th stage for further removal of biuret. The permeate of thelast stage represents the purified solution produced by the overall ROprocess.

The first retentate may be processed through a second set of one or moreRO stages. The retentate of the last RO stage of said second seteventually forms a biuret-rich stream.

The retentate stream(s) from RO stages of said first set together withthe permeate stream(s) taken from the RO stages of said second set maybe recycled to the inlet of the first RO stage together with the inputsolution.

The input aqueous solution of urea, after its withdrawal from therecovery section, may be stored in a urea solution tank. In accordancewith this embodiment, the urea aqueous solution which is subject toreverse osmosis for removal of biuret is taken from said tank.

Preferably the input solution contains at least 25% urea. At thisconcentration the osmotic pressure calculated for the binary mixturewherein water is the solvent and urea is the solute, is significantlyhigher than 100 bar. Preferably, urea and water account together for atleast 90% wt of the solution, more preferably at least 95% wt. When theinput solution is the solution obtained from a recovery section of aurea plant, it contains preferably 60% to 90% of urea by weight. Thebalance is predominantly water and includes biuret and possibly otherimpurities.

In some embodiments a step of flash or pre-evaporation at asub-atmospheric pressure of the urea aqueous solution may be performedbefore said solution is stored in the tank. The term sub-atmosphericpressure denotes an absolute pressure of less than 1 bar, preferablyless than 0.5 bar. This preliminary step of flash or pre-evaporation isadvantageous to maintain a low concentration of carbonates in thesolution stored in the tank. Preferably the carbonates are kept below0.2% by weight and more preferably below 0.1%.

A low carbonate content in the solution may be helpful to maintain theosmotic pressure of the concentrate below a desired level, e.g. lessthan 70 bar or preferably less than 40 bar. It should be noted in thisrespect that a semi-permeable membrane is typically highly selective tosalts. For example in a multiple-stage RO process the salts contained inthe input solution may be almost completely removed in the first stage.For this reason, a high content of salts (e.g. carbonates) in the inputsolution may lead to undesirable increase of the osmotic pressure.

The low-biuret purified solution which is obtained after the reverseosmosis process may have a content of biuret half of the inputconcentration.

The purified solution obtained after the reverse osmosis process may besubject to a step of evaporation to remove water. Particularlypreferably, said step of evaporation obtains a highly concentratedsolution or urea melt suitable for granulation or prilling.

The solid product resulting from the low-biuret purified solution whichis obtained after the reverse osmosis process may contain no more than0.7% wt of biuret.

A biuret-rich stream (retentate) produced in the RO process may be usedas raw material for obtaining a secondary product based on biuret, forexample feed-grade biuret. This biuret-rich stream may also be recycledto a urea plant, e.g. added to a condenser of a recovery section to helpcondensation of vapours containing ammonia and CO₂. If this is the case,the flow rate of the retentate recycled to the recovery section ispreferably not greater than 10% of the flow rate of the aqueous solutionsubject to the reverse osmosis purification process

In the various embodiments of the invention, the aqueous urea solutionwhich is subject to the RO process of purification may be regarded as abinary mixture wherein the biuret is a solute and the water-urea mixtureis a solvent. That is to say, the water-urea mixture can be regarded asa solvent of the biuret. Also in case the input solution containssignificant amounts of carbonates and/or ammonia, this approach is stillapplicable considering biuret and carbonates as the solute and themixture of water, urea and ammonia as the solvent.

The osmotic pressure can be calculated using the following formula:

$\prod{= {- {\frac{RT}{v_{solvent}} \cdot {\ln\left( a_{solvent} \right)}}}}$

wherein: Π is the osmotic pressure (Pa);

R is the universal gas constant (J K⁻¹ mol⁻¹);

T is the absolute temperature (K);

v_(soivent) is the molar volume of the solvent (m³ mol⁻¹);

a_(soivent) is the (dimensionless) activity of the solvent.

For a diluted solution, the activity of the solvent can be approximatedto the molar fraction of solvent.

It has to be noted that the coefficient of rejection of carbonatespossibly dissolved in the input solution is significantly greater thanthe coefficient of rejection of the biuret, due to dissociation of thecarbonates. The term carbonates denotes salts of the carbonic acid.

The invention is applicable to all known processes and plants for thesynthesis of urea. A preferred application is to a stripping process,most preferably a CO₂-stripping process.

The invention, in its various embodiments, allows produce a low-biuretsolid urea or low-biuret urea solution without the cost and complicationof a crystallization section.

The invention is now further elucidated with reference to preferredembodiments and the accompanying figures.

DESCRIPTION OF FIGURES

FIG. 1 is a scheme of a process for producing urea in an embodiment ofthe invention.

FIG. 2 is a scheme of a multiple-stage reverse osmosis section which canbe used to implement the invention.

DETAILED DESCRIPTION

Referring to FIG. 1 , a urea synthesis plant UP produces a urea aqueoussolution 1 of urea. Said solution 1 is taken from a recovery section ofthe plant UP. The plant UP more in detail may include a high-pressuresynthesis section—e.g. a CO₂-stripping synthesis section—and alow-pressure recovery section from which the solution 1 is obtained.

Said solution 1 is stored in a urea solution tank T. The solution 2taken from said tank 2 is sent to a reverse osmosis section RO includinga membrane package which performs a reverse osmosis process to removebiuret from said solution 2.

A low biuret urea solution 3 is obtained from the section RO. Thislow-biuret solution 3 is sent to an evaporation section EV where wateris removed and a highly concentrated solution 4 is obtained. This highlyconcentrated solution 4 is processed in a finishing section FIN toobtain solid urea U in the form of prills or granules.

A biuret-rich solution 5 is also produced in the section RO. Saidsolution 5 contains the biuret removed from the input solution 4 and hastypically more than 1% biuret. Said solution 5 is recycled to the plantUP. A preferred use of the solution 5 in the plant is sending thesolution 5 into a condenser of ammonia and CO₂ vapours.

In another interesting application, water may be added to the stream 3to produce a urea solution for use in SCR for removal of NOx.

FIG. 2 illustrates an exemplary embodiment of the section RO section.

An input solution F (e.g. the solution 2 of FIG. 1 ) is sent to a firstreverse osmosis stage RO-1 together with internal recycle streams 20,21. The stage RO-1 therefore receives a mixed stream 22 and produces afirst permeate P1 and a first retentate R1.

The first permeate P1 is processed in a set of stages RO-1.1 and RO-1.2wherein the permeate is progressively purified. Particularly thepermeate P2 of the stage RO-1.1 is further purified in the stage RO-1.2to produce a permeate P which is a first output of the process (e.g. thestream 3 of FIG. 1 ).

The first retentate R1 is processed in a set of stages RO-2.1 to RO-2.3.The retentate of each stage forms the input of the subsequent stage. Theretentate R of the last stage RO-2.3 is another output of the process,for example the stream 5 of FIG. 1 .

The stream P has the lowest amount of biuret whilst the stream R has thehighest. The permeate streams of the stages RO-2.1 to RO-2.3 and theretentate streams of the stages RO-1.2 and RO-1.3 are streams withintermediate content of biuret; they can be recycled to the inlet of thefirst stage RO-1 via lines 20, 21 as shown in FIG. 2 .

For example, in a preferred embodiment the streams of FIG. 2 have thefollowing flow rates (m³/h) and mass fraction of biuret w_(B).

Stream m³/h W_(B) F 95 0.50 22 290 0.77 P1 194 0.50 P2 107 0.34 P 880.25 R 7 4.0 20 90 1.05

The invention achieves the above mentioned goals of providing acost-effective process for removing biuret from urea solutions andproduce low-biuret urea.

1. A process for purifying a urea-containing aqueous stream comprising astep of removing biuret from the urea-containing stream by reverseosmosis, wherein the urea-containing stream is an aqueous solution ofurea obtained from a recovery section of a urea plant, preferablycontaining 60 to 90 wt % of urea.
 2. The process according to claim,wherein the reverse osmosis is performed with a thin-film compositemembrane.
 3. The process according to of claim 1, wherein the reverseosmosis is performed with the urea-containing stream having atemperature of 60° C. to 90° C., preferably 70° C. to 80° C.
 4. Theprocess according to claim 1, wherein the reverse osmosis is performedwith one or more reverse osmosis stages in a cascade.
 5. The processaccording to claim 4, wherein the difference of pressure across the oreach stage of the reverse osmosis process is 30 bar to 70 bar.
 6. Theprocess according to claim 1, wherein the process produces a purifiedsolution with a content of biuret lower than the input solution andhaving the same or substantially the same water to urea ratio as theinput solution.
 7. The process according to claim 1, wherein the aqueousurea solution contains at least 25% wt of urea.
 8. The processcomprising: reacting ammonia and carbon dioxide under urea-formingconditions and urea synthesis pressure in a urea synthesis section toform a urea-containing reaction effluent; processing saidurea-containing reaction effluent in a recovery section, including oneor more recovery stages at a recovery pressure lower than said ureasynthesis pressure, to remove unreacted ammonia and carbon dioxide fromthe reaction effluent and obtain a urea aqueous solution; purifying saidurea aqueous solution to remove biuret with a process according toclaim
 1. 9. The process according to claim wherein all or some of theurea aqueous solution withdrawn from the recovery section is stored in aurea solution tank and the urea aqueous solution subject to said reverseosmosis is taken from said tank.
 10. The process according to claim 9,including a step of flash or pre-evaporation at a subatmosphericpressure of the urea aqueous solution before said solution is stored insaid tank.
 11. The process according to claim 9, including a step ofevaporation to remove water from a purified urea solution obtained withthe reverse osmosis process.
 12. The process according to claim 8,wherein the osmosis process produces a permeate, which is a purifiedlow-biuret urea-containing solution, and a retentate, which containsbiuret removed from the input solution, and wherein at least part ofsaid retentate is recycled to the recovery section.
 13. The processaccording to claim 12, wherein the retentate recycled to the recoverysection is used in a condensation step of CO2- and ammonia-containingvapours as a means to improve condensation.
 14. The process according toclaim 12, wherein the flow rate of the retentate recycled to therecovery section is not greater than 10% of the flow rate of the aqueoussolution subject to the reverse osmosis purification process.
 15. Theprocess according to claim 8, further including the production of biuretor feed-grade biuret and wherein at least part of said retentate is usedto produce said biuret or said feed-grade biuret.
 16. The processaccording to claim 8, wherein urea is synthesized with a strippingprocess, preferably a CO2-stripping process.
 17. A plant for thesynthesis of urea including: a urea synthesis section adapted to produceurea by reacting ammonia and carbon dioxide at a urea synthesispressure; a recovery section arranged for processing a urea-containingreaction effluent produced in the urea synthesis section, the recoverysection operating at one or more recovery pressure(s) lower than saidurea synthesis pressure, to remove unreacted ammonia and carbon dioxidefrom the reaction effluent and obtain a urea aqueous solution; and apurification section arranged to remove biuret from the urea aqueoussolution obtained in the recovery section, said purification sectionincluding one or more reverse osmosis stages where biuret is removedfrom the solution by means of a reverse osmosis process.
 18. The plantaccording to claim 17, wherein the one or more reverse osmosis stagesare upstream a concentration section and a finishing section for theproduction of solid urea, so that the removal of biuret by reverseosmosis is performed before the solution is concentrated by removingwater.
 19. The process according to claim 3, wherein the reverse osmosisis performed with the urea-containing stream having a temperature of 70°C. to 80° C.
 20. The process according to claim 5, wherein thedifference of pressure across the or each stage of the reverse osmosisprocess is 35 bar to 50 bar.
 21. The process according to claim 5,wherein the difference of pressure across the or each stage of thereverse osmosis process is 40 bar or around 40 bar.